The present invention relates to intravascular implants. In particular, the present invention relates to stent devices to deliver therapeutic agents such as radioisotopes or drugs.
In the last several years, minimally invasive surgical procedures have become increasingly common. Minimally invasive procedures such as percutaneous transluminal coronary angioplasty (PTCA) are widely utilized. A PTCA procedure involves the insertion of an angioplasty balloon at the distal end of a catheter to the site of a stenotic lesion. Prior to treatment, the stenotic lesion is bulky and at least partially blocking the coronary artery at issue. Once advanced, the balloon is inflated compressing the stenosis and widening the lumen in order to allow an efficient flow of blood through the lumen.
Following PTCA and other stenotic treatment procedures, a significant number of patients may experience restenosis or other vascular blockage problems. These problems are prone to arise at the site of the former stenosis.
In order to help avoid restenosis and other similar problems, a stent may be implanted into the vessel at the site of the former stenosis with a stent delivery catheter. A stent is a tubular structure which is delivered to the site of the former stenosis or lesion and compressed against vessel walls thereat, again with a balloon. The structure of the stent promotes maintenance of an open vessel lumen. The stent can be implanted in conjunction with the angioplasty.
In addition to stent implantation, radiotherapy and drug delivery treatments have been developed and applied to the site of the former stenosis following angioplasty. Generally such treatments can aid in the healing process and significantly reduce the risk of restenosis and other similar problems.
In some cases, stent implantation may be combined with drug delivery or radiotherapy. For example, a stent may be drug loaded or radioactive. A stent with a therapeutic agent may be delivered to the physician about the stent delivery catheter (and with a removable shield if the stent is radioactive).
However, delivery of a therapeutic treatment throughout the site of the former stenosis is problematic. The level of uniformity in the delivery of a therapeutic agent to the injured area is dependent upon the particular stent configuration. For example, in the case a radioactive stent, the radioactive stent may have hot spots and cold spots of uneven levels of radioactivity. This is because the stent is made up of struts having radioactivity and window cells having no physical structure or radioactivity (or drug in the case of a drug delivery stent). Therefore, therapeutic agent throughout a particular stent configuration is dependent upon the strut and window cell distribution throughout that stent. Therefore, therapeutic variability results.
For example, in the case of a radioactive stent, if about 20 Grays (Gy) of radiation, as measured from 1 mm of tissue depth, are to be delivered to a vessel portion to be treated, a wide range of radiation delivery will actually occur. That is, due to the radioactive stent configuration, a non-uniform delivery, ranging from about 5 Gy to about 25 Gy is more likely delivered to the vessel portion to be treated. Due to limitations of the prior art a range of at least about 20 Gy will be delivered by a radioactive stent throughout the vessel portion to be treated in the given example. As a result, certain portions of the vessel will receive significantly more or significantly less radiation than intended. Such a variability in delivery could lead to underdose failing to reduce the risk of restenosis in certain portions of the vessel, or overdose potentially causing further vascular injury to other portions of the vessel. This variability results regardless of the therapeutic agent to be delivered.
Additionally, certain therapeutic agents are delivered to avoid a phenomenon known as xe2x80x9cedge restenosisxe2x80x9d. Edge restenosis is prone to occur near stent ends.
Even though a stent is structurally configured to maintain the patency of a vessel lumen, edge restenosis is prone to occur with the use of radioactive stents. Edge restenosis involves the formation of vascular overgrowths in vascular areas immediately adjacent radioactive stent ends, generally within about 2 mm of each radioactive stent end. Edge restenosis is a result of delivery of a sub-threshold level of radiation to the vascular areas immediately adjacent the radioactive stent ends. These vascular areas are near or within the site of the former stenosis. They include vasculature likely to be diseased, or subjected to a recent trauma such as angioplasty. When a sub-threshold level of radiation, between about 2 Grays and about 10 Grays, as measured at 1 mm of tissue depth, reaches such vulnerable vascular areas, stenotic overgrowths may actually be stimulated. These overgrowths result in narrowed vessel portions near stent ends giving an appearance of a candy wrapper crimped around the ends of the stent. Thus, this effect is often referred to as the xe2x80x9ccandy wrapperxe2x80x9d effect.
The occurrence of the candy wrapper effect is likely when a radioactive stent is used. This is because the intensity of radiation decreases as the source of the radiation, the radioactive stent, terminates at its ends leading to a drop of in radiation levels at vessel portions adjacent its ends. Thus, a sub-threshold radiation delivery is likely to occur near the radioactive stent ends.
As indicated, heretofore, the level of therapeutic uniformity or focus any particular stent has been able to deliver has been dependent upon that stent""s configuration with respect to strut and window cell distribution. However, a stent structure (i.e. strut layout) which physically promotes maintenance of an open vessel lumen may be of a particular configuration which is not necessarily best suited for a more uniform delivery of a therapeutic agent. Additionally, this stent configuration may fail to avoid an unintended xe2x80x9ccandy wrapperxe2x80x9d effect in which portions of the vessel adjacent the stent become narrowed.
An embodiment of the present invention provides a stent having a variable stent surface area per unit length. The variable stent surface area is used to accommodate a therapeutic agent.
Another embodiment of the present invention provides for a stent having an end and a variable stent surface area per unit length to accommodate a therapeutic agent. A decreased level of therapeutic agent in provided at the end.
An embodiment of the present invention provides for a stent having an end and a variable stent surface area per unit length to accommodate a therapeutic agent. An increased level of therapeutic agent in provided at the end.
In an embodiment of the invention a method of vessel treatment utilizing a stent with a variable stent surface area is provided. A therapeutic agent is disposed on the stent surface area to provide a patterned distribution of the therapeutic agent.
In another embodiment of the invention a method of stent manufacture is provided where indentations are cut into a surface of a stent. A therapeutic agent is disposed on the surface of the stent.
In another embodiment of the invention a method of stent manufacture is provided where struts of the stent are cut of increased thickness to provide a variable stent surface area. Therapeutic agent is disposed on the variable stent surface area.